Sub-adults in their 3rd and 4th summer (fourth calendar year and fifth calendar year respectively) show serious overlap in characteristics. Best clues for ageing these birds is the combination of bare part coloration and age of the primaries and secondaries. Nevertheless, although white tips on the primaries and a mirror on p10 may be expected in 4cy birds, this is not always the case. The same applies for bare parts, which may look very immature in one bird, very adult-like in another while you got the feeling these two have to be born in the same year. In general, sub-adults show:

very small white tips on the old outer primaries (but beware wear),

a very limited white mirror on the old p10 and absence of a mirror on p9,

isolated black markings on old, worn rectrices,

brown centred coverts, often found in the carpal edge (outer lesser coverts),

vermiculation on (central) tertials or an accentuated black centre in the tertials, which may have the white tips confined to the inner web,

immature colouration of bare parts,

secondaries show brownish centres with a zigzag pattern along the trailing edge.

Arrested and suspended moult

In other sections, much as been said about suspended or arrested moult and by June such birds can be found at the IJmuiden beach and in the Maasvlakte colony (both the Netherlands). For problems related to correct ageing and allocation to specific sub-species, the discussion page can be found here.

Breeding birds are not necessarily fully mature adults, as both Herring and Lesser Black-backed Gull may breed as sub-adults. Chabrzyk & Coulson (1976) found that 14% of Herring Gulls start breeding as 4cy birds (also see Drost et al., 1961). In the Herring Gull we used black on the primary coverts as a feature to separate between fully mature birds and sub-adults (see thumbnails). In the results, we will distinguish between sub-adult and adult birds (showing clean grey primary coverts, see thumbnails) when results need separation for age.

Fully mature Lesser-black-backed Gulls may still show obvious black markings on the primary coverts (personal observations based on ringed birds). We have illustrated all greater primary wing coverts of known-age birds HERE. We classified sub-adults as obviously immature birds, considered 4cy and 5cy, with extensive brown-black on the primary coverts. By doing so, we may have excluded delayed 6cy or older birds and included advanced 4cy or 5cy birds. However, in the absence of any other safe criterion, primary covert markings were considered the most appropriate way to distinguish full adult from sub-adult birds (see plate 3). Sixty-two Lesser Black-backed Gulls were scored ‘sub-adult’: 43 males and 18 females. The sex-ratio for the total sample is reverse: 443 males and 515 females.

Based on ring recoveries, the first breeding birds at Moerdijk (early 1980s) probably originated from the vast colonies at Maasvlakte and Dintelhaven (51.57N 04.01E). In recent years we have also found birds which were born in the recently established colony of Zeebrugge, Belgium (51.21N 03.11E). Ring readings show that nowadays the Moerdijk colony mainly hosts locally-reared birds.

In 2005-2006 we ringed and measured 315 breeding Herring Gulls (165 males and 137 females, 13 birds not sexed) and 1038 Lesser Black-backed Gulls (460 males and 535 females, 43 birds not sexed). Wingspan in the Herring Gull population ranges from 385 – 465 (average 427) and ranges from 385 – 462 (average 426) in Lesser Black-backed Gulls (table 1). We trapped 113 birds of known age (35 Herring Gulls and 77 Lesser Black-backed Gulls). Known age varied from 4cy to 19cy. The oldest known bird in our sample is Dutch HG mS0, born in 1987 (i.e. 19 CY) and caught in the Moerdijk colony in 2005.